Crop assign

[pluskid]

Depends on dmlc/mshadow#172

Added a _crop_assign op to replace legacy _copy_slice_to and _assign_slice_from operators.

I did not expose this to __setitem__ and __getitem__ as this will make copy, while our existing get item functions shares memory.

@piiswrong

[piiswrong]

This can be safely exposed to setitem and return None to disable chain assign.
we can discuss how to do it for getitem later. My opinion is it should return a copy that's marked as read only

[pluskid]

Exposed __setitem__